1. Field of the Invention
The present invention relates to a method for replacing a lens having refractive power in a projection optical system that projects a pattern of an original body (such as a mask) illuminated with exposure light onto a substrate with an alternative lens, and to a method for manufacturing the alternative lens.
2. Description of the Related Art
Projection exposure apparatuses have been used to manufacture devices such as semiconductor devices and liquid crystal devices. Further, according to demands for finer circuit patterns, the resolution of such projection optical systems has been improved.
On the other hand, the allowance for contamination of the lens in the projection optical system has considerably decreased in order to maintain high resolution. This is because, when a foreign substance adheres to a surface of the lens in the projection optical system, transmittance of the lens decreases, uneven illuminance occurs, and resolution deteriorates.
Accordingly, a replaceable optical element is used in the projection optical system, and is replaced when contaminated. More specifically, a replaceable parallel plate is placed at a side of the projection optical system closest to a wafer. Even when mounting error of the parallel plate is caused by replacement (even when the parallel plate is displaced), there is little influence on aberration of the projection optical system.
The parallel plate is also replaced in order to reduce aberration of the projection optical system due to temporal change. A procedure for replacing the parallel plate will now be described with reference to FIG. 6 serving as a flowchart.
When performance of an exposure apparatus decreases at the user's site, first, a wavefront aberration of a projection optical system U installed in the exposure apparatus is measured (Step S61). Then, a replaceable parallel plate is taken out of the projection optical system U (Step S62). The parallel plate is returned to a manufacturing plant of a manufacturer of the exposure apparatus, and a surface shape of the parallel plate is measured at the manufacturing plant (Step S63). On the basis of the measured surface shape and the wavefront aberration measured in Step S61, a surface shape (aspherical surface) of the parallel plate that can reduce the wavefront aberration is calculated (Step S64), and an alternative parallel plate is processed so as to have the calculated surface shape (Step S65). The processed alternative parallel plate having a new surface shape is delivered to the user, and is then mounted in the projection optical system U that is installed in the exposure apparatus (Step S66).
Alternatively, it has been proposed that an alternative optical element is used to reduce assembly error of a projection optical system (see Japanese Patent Laid-Open Nos. 2006-287140, 2005-268412, and 2003-29117).
In recent years, immersion exposure apparatuses have come onto the market. In an immersion exposure apparatus, liquid (for example, pure water) fills the space between a final lens of a projection optical system and a wafer. For this reason, there is a possibility that contaminants from a resist applied on a surface of the wafer adhere to the final lens in contact with the liquid, and that a surface of the final lens will thereby be subjected to fogging, which does not occur in dry-type exposure apparatuses.
Compared with dry-type exposure apparatuses, the final lens is very close to the wafer in immersion exposure apparatuses. For this reason, near the wafer, exposure light from an exposure light source is collected, and illuminance increases. As a result, the refractive index of a lens material of the final lens may be easily changed, and this may deteriorate optical performance of the projection optical system.
Accordingly, it is necessary to periodically replace the final lens in order to remove the influence of fogging and changes in the refractive index of the final lens on the optical performance of the projection optical system.
In order to increase the numerical aperture NA of the projection optical system in the immersion exposure apparatus, the final lens of the projection optical system needs to be a convex lens having a positive refractive power. If an optical element to be replaced is a parallel plate, mounting error caused by replacement has little influence on the optical performance of the projection optical system, as described above. In contrast, mounting error (for example, displacement in the direction of the optical axis of the projection optical system) of a lens having refractive power significantly affects the optical performance of the projection optical system. Therefore, it is necessary to mount the final lens in the immersion exposure apparatus with very high precision.
When the projection optical system has a high resolution, the difference in shape (curvature, thickness, etc.) between exchanged final lenses, that is, the current final lens and an alternative lens thereto, has a great influence on the optical performance of the projection optical system. Therefore, it is necessary to process the alternative lens in the immersion exposure apparatus with very high precision.
Further, when the numerical aperture NA of the projection optical system is large, the final lens is designed to have a small radius of curvature, that is, have a large refractive power in order to reduce the outer shape of the projection optical system. Such a lens having a large refractive power is thick. As the thickness of the lens increases, aberration caused in the projection optical system because of decentration of the lens and nonuniform refractive index of the lens material increases. In the case of a parallel plate, the influences of decentration and nonuniform refractive index can be reduced by decreasing the thickness of the parallel plate. In contrast to the parallel plate, it is difficult to decrease the thickness of the final lens in the immersion exposure apparatus, and therefore, the influences of decentration and nonuniform refractive index are difficult to reduce.